The field of this invention relates to a polycondensation process and apparatus for the manufacture of polymers from a mixture of starting materials using a high-temperature system wherein the polymerization is conducted in a dispersion which is in a turbulent flow or drop-flow regime. The dispersion is passed through a reaction zone in which heat is introduced to polymerize the starting materials and form a polycondensate. The process is suitable for the manufacture of polyesters, polycarbonates, polyarylates, polyestercarbonates, polyamines, polyamide-imides, polyamides, and other condensation polymers. This novel process is applicable to any polycondensation reaction provided the mixture of the starting materials is essentially homogeneous. Also, the mixture of starting materials, or the salt or prepolymer which is formed upon combination of the starting materials, must be stable under the high reaction temperatures and be capable of forming a dispersion in a vapor phase.
A process for the vapor-phase condensation polymerization of aromatic polyamides is disclosed by Shin in U.S. Pat. No. 4,009,153 (1977). In this process vaporized monomers are diluted with an inert gas and then mixed and reacted in a reaction zone heated in the range of 150.degree. C. to 500.degree. C. One type of reactor described is a so-called "space reactor" in which the residence time is low, preferably less than 5 seconds. The polymerization is presumed to occur in space and not on the reactor walls. This process has a number of disadvantages. First, the reaction zone must be maintained at a high temperature in order to keep the monomers in vapor phase. These high temperatures limit the polymers which can be produced to those which will not decompose. Second, there is only about 20 percent reaction of the monomers per pass through the reaction zone (see Example 6). This means that there must be a large monomer recovery and recycle section in the process which is commercially undesirable.
A large number of processes have been disclosed in the prior art which attempt to provide a continuous method for preparing polycondensation polymers. U.S. Pat. No. 4,060,517 of Mertes et al. (1977) provides a review of a number of these processes and is incorporated herein by reference. The prior art processes which involve a flashing step can be divided into two categories. In the first category, a mixture of starting materials is heated under pressure and then flashed while attempting to maintain isothermal conditions. In the second group of processes, there is an adiabatic release of pressure with accompanying release of all the volatile components.
Among the prior art processes in the first category is that of Taul et al., in U.S. Pat. No. 3,027,355 (1962), who disclose a method for removing water from a concentrated, polyamide-forming condensate. This method involves flashing the condensate through nozzles directed so as to force impingement of the condensate against an extensive, heated metal surface to minimize the heat loss of the condensate. The foam-like product then flows downward over the heated surfaces into a heated processing vessel in which the polymerization is completed.
Carter, in U.S. Pat. No. 3,193,535 (1965) and Canadian Patent 800,061 (1968), describes a process in which a solution is subjected to a gradual reduction in pressure as it passes through a heated tube in order to remove water vapor. There is no abrupt or sudden fall in pressure along the tube. The cross-section of the heated tube ordinarily increases stepwise throughout its length to provide the continual reduction in pressure. Heckert, in U.S. Pat. No. 2,689,839 (1954), describes a method of gradually reducing pressure in molten polymer by passing it through a long tube of successively increasing diameter. Parnell, in U.S. Pat. No. 3,257,173 (1966), Griffiths, in U.S. Pat. No. 3,258,313 (1963), and Iwasyk, in U.S. Pat. No. 3,948,862 (1976), describe modifications in this process and apparatus. As set forth in column 3, line 8 of Iwasyk, this process provides a two-phase annular flow in the long tube or "flashing reactor".
Taylor, in U.S. Pat. No. 2,361,717 (1944), discloses a process in which the pressure release is carried out semi-adiabatically and semi-isothermally. The heated reaction mixture is passed through a valve and flashed into a heated coil. The resulting mixture of steam and polymer is passed through the coil and into a trap from which the molten polymer is withdrawn. The residence time of the polymer in the steam-flashing stage is about 1 to about 5 minutes, as taught in the first column of page 3, lines 23 through 34. Additionally, it can be determined from the examples that the specific mass flow rate (as defined hereinbelow) is about 41 pounds per hour per square inch.
The second category of processes which utilizes adiabatic release of pressure is exemplified by Mertes et al. (cited hereinabove), Clemo et al., U.S. Pat. No. 3,185,672 (1965) and Doerfel et al., U.S. Pat. No. 4,049,638 (1977). Clemo discloses a process for the manufacture of polyamides which involves pumping a hot aqueous salt solution through a pressure tube at polyamide-forming temperatures under a pressure sufficient to prevent the evolution of steam until a degree of polymerization of about 1.15 to 1.37 is obtained and then the solution is adiabatically flashed into a chamber at atmospheric pressure. As taught in col. 3, lines 47 through 55, this flashing is accomplished by passing the prepolymer solution through a jet or a valve or other spraying device to form small droplets. The droplets of the spray rapidly contact the surface of the chamber, the molten mixture of salt and polyamide flows down the sides of the heated chamber and the melt collects in the bottom of the chamber to be forwarded by a pump to another vessel in which the process of polymerization is completed. This adiabatic flashing step serves as a method of pressure reduction and also water removal in the process.
Mertes et al. disclose a process for preparing polyamides in which a mixture of starting materials is heated in a first reaction zone until the conversion is at least 80 percent. The product is then passed to a second zone where the pressure is adiabatically released. The polycondensation mixture is then transferred into a heat exchanger such as a tube bundle heat exchanger and rapidly heated to between 220.degree. C. and 330.degree. C. in less than 5 minutes, preferably in less than one minute, to evaporate the bulk of water. The resulting product is then passed into a post-condensation zone, for example, a self-purging, twin-screw reactor and additional condensation reaction is carried out to give the desired molecular weight.
Doerfel discloses a process for producing polyamides in which reactants are contacted in a precondensation zone until polymerized to a conversion of at least 70 percent. This mixture is then adiabatically flashed into a heat exchanger such as a cascade-connected tubular heat exchanger. The material leaving the heat exchanger passes into another reaction vessel where volatiles are removed and the molecular weight is increased. In this process, the vapor flow through the heat exchanger is not maintained at a high enough level to achieve and maintain the condensate as a dispersion, i.e., the condensate contacts the heat exchanger as a liquid film.
Other patents which might be of interest relating to flashing include: Brignac, U.S. Pat. No. 3,501,441 (1970) and U.S. Pat. No. 3,300,449 (1967); Sovereign, U.S. Pat. No. 3,218,297 (1965); Lodge, U.S. Pat. No. 3,278,494 (1966); Jaswal et al., U.S. Pat. No. 3,900,450 (1975); Hawkins, U.S. Pat. No. 3,195,613 (1965); and Coggeshall, U.S. Pat. No. 3,260,703 (1966).
Other patents which disclose tubular reactors include: Parker et al., U.S. Pat. No. 3,241,926 (1966); Brill et al., U.S. Pat. No. 3,192,184 (1965); Wise et al., U.S. Pat. No. 2,018,771 (1935); Girantet et al., U.S. Pat. No. 3,600,137 (1971); Hellemanns et al., U.S. Pat. No. 3,880,921 (1975); Greene, U.S. Pat. No. 4,221,763 (1980); Pinney, U.S. Pat. No. 3,960,820 (1976); and Tate, U.S. Pat. No. 3,296,217 (1967).
The prior art systems require significantly longer residence times than the instant invention. This limits the temperature at which the processes can be operated since the polymer may degrade at the longer residence time. The lower operating temperatures also limit the polymers which can be prepared by the prior art processes; additionally, many of the designs are complex and expensive. Consequently, a process is needed having a simple design which allows a short residence time of material in the system and can therefore operate at much higher temperatures. Also, a process is needed which can be used to prepare a wide range of condensation polymers having different melt viscosities without major process changes.